The invention relates generally to Smith-Purcell free-electron lasers, such as orotrons, ledatrons, and diffraction radiation generators. More Particularly, the invention relates to apparatus for electronically tuning orotrons and orotron-type devices over a wide frequency band.
In an orotron or similar device, a ribbon-like electron beam is directed over the surface of a reflecting diffraction grating and radiates into a mode of an open resonator formed by two mirrors, one of which is partially covered by the diffraction grating. The radiation generated by the beam-grating interaction (the so called Smith-Purcell effect) is fed back into the beam and bunches the electrons. If the proper conditions of synchronism between the electron velocity and the phase velocity of an evanescent wave travelling along the grating are met, coherent radiation results. The electron velocity is controlled by the cathode-to-grating voltage, and the mirror separation determines what type of RF mode can be set up to resonate in the open resonator form by the two mirrors.
Various orotrons and orotron-like devices, such as the 50-to-75 GHz orotron developed at the Harry Diamond Laboratories (HDL), Adelphi, MD, are tunable over a wide frequency range. However, in the past, the tuning of such orotrons was done by first changing the mirror separation mechanically and then adjusting the beam accelerating voltage (grating-to-cathode voltage), a tedious and relatively slow procedure.